A very large and well known financial firm I was affiliated with mixed ITIL and Six Sigma to help drive justification and identify areas of focus for ITIL implementation. They used ITIL to create a large landscape to look in, for improvement. They then used Six Sigma to find very specific areas, within that broad landscape, that needed the most immediate attention, along with using it to create real justifications for improving such areas. I don't know if I can say that it was a perfect way to mix the two but I can say that they were able to achieve some definite wins.